Add a PHID to Changesets

Summary:
Ref T13605. Changesets currently have no PHID, which limits their ability to use standard API infrastructure.

Give them a PHID, since there's no reason they don't have one other than their age.

Test Plan:

• Ran migrations, saw PHIDs populated.
• Created new changesets, saw PHIDs.

Subscribers: PHID-OPKG-gm6ozazyms6q6i22gyam

Maniphest Tasks: T13605

Differential Revision: https://secure.phabricator.com/D21557